Method of manufacturing a superconductive cable

ABSTRACT

A superconductive conductor or cable comprising a core, which comprises at least one string of a ceramic, superconductive material, and where the core is encapsulated by a metal cap. In order to manufacture a conductor or a cable with an encapsulation, and in which it is possible as well during the manufacturing process as under the operation to maintain a controlled atmosphere around the superconductive core, at least one layer of not sintered, ceramic powderous material is provided between the cap and the core, which material has a higher sintering temperature than the superconductive material in the core. The superconductive core may be sintered for formation of the superconductive ceramics by placing in the tubular metal cap a starting material in powderous form, and shaped as a core in the other ceramic powder material and subsequently forgeing the the metal cap with its content, preferably by swaging at an ambient temperature, which is below the sintering temperature of the core.

BACKGROUND OF THE INVENTION

The present invention relates to a superconductive conductor or cablecomprising a core, which comprises at least one string of a ceramicsuperconductive material, and where the core is encapsulated by ametallic cap. Furthermore the invention relates to a method ofmanufacturing such a conductor or cable.

Ceramic compounds of the type, which are often designated by, theformula Y-Ba-Cu-O have metals in oxidized form and exhibit electricsuperconductive characteristics at a substantially higher temperaturethan that of conventional superconducting materials. Because of therelatively high marginal temperature of the superconductivecharacteristics, this type of compounds has become more interesting foruse in a greater variety of industrial application. The ceramiccompounds form a whole group of materials, in which copper and oxygenseem to be the only required materials, as they, in addition to orinstead of the metals yttrium and barium, may also comprise e.g.scandium, various lanthanides, calcium or strontium. Superconductorscomprising other materials, e.g. bismuth and/or thallium are also known,as such materials may completely or partly replace the abovementionedmetals. Furthermore, it is known that the superconductivecharacteristics may be improved in some cases, if fluorine replaces partof the oxygen.

The use of the superconductive materials is restricted by the fact thatthe manufacture of these conductors in sufficient lengths and with asufficiently homogeneous structure for the industrial application offabricating cables is difficult and costly. An essential problem in thisconnection is that in the manufacturing process a solid-reaction betweensuitable compounds, e.g. oxides, carbonates or oxalates must take place;that the reaction must take place under a controlled set of temperature,and that the composition of the surrounding atmosphere may be ofconsiderable importance. In particular, a high partial pressure ofoxygen may be necessary. Furthermore there is a considerable risk thatthe reaction product thus formed may be unstable and liable to split offe.g. oxygen. It may, therefore, be necessary to surround thesuperconductive core, also under later operation, by a controlledatmosphere.

These problems are discussed in Fujikura Technical Review, No. 17, Feb.(Tokyo JP), H. Osani et al, pages 1-4, in which different materials forthe metal cap have been tested, and in Advanced CeramicMaterials--Ceramic Superconductors, Volume 2, No. 38, Special Issue,Jul. 1988, (Westerville, Oh., US), R. W. McCallum et al pages 388-400,in which silver is identified as a preferred material for encapsulationbecause of its permeability to O₂, and in which addition of silveroxides inside the cap has been suggested as a means for controlling theatmosphere inside the cap during the sintering process of thesuperconductive material.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a conductor or a cablewith an encapsulation, and in which it is possible both during themanufacturing process and during the later application to maintain acontrolled atmosphere around the superconductive core.

This object is achieved by means of a conductor or a cable of theabovementioned kind, the cable being characterized by the presence of aceramic powderous material between the cap and the ceramic core of thecable.

Due to the fact that the ceramic, superconductive core is surrounded byan only partially sintered ceramic powderous material, a flow of gas anddiffusion in the pore volume, which exists in such material, can takeplace. On account of the higher sintering temperature of the surroundingmaterial, it will remain porous also during various manufacturingprocesses, to which the cable will be subjected in the productionprocess, and it is therefore ensured that the surrounding material willretain its permeability to gas.

According to the invention it is preferable that the surrounding layerhas a high temperature stability, and preferred embodiments of theinvention hot and cold resistant oxides can be mentioned, especiallyoxide ceramics preferably MgO, Al₂ O₃ or ZrO₂.

In order to be able to maintain a controlled atmosphere around theconductive core, it is preferable according to the invention that thecable comprises elements for introducing a maintaining or regeneratinggas, e.g. O₂ in the non-sintered material.

The invention further relates to a method of manufacturing an electricsuperconductive conductor or cable, in which method one or morematerials, which under specified circumstances can be madesuperconductive, are placed in the form of at least one continuousstring in a metallic, tubular, closed cap, which thereafter is subjectedto a forging process producing a deformation of the closed cap. Thedeformation can be caused by swaging, hammering or causing an explosionin the cap producing a tightly confined materials reduction of the crosssection surrounding the.

From Japanese Patent No. 6113663-A, dated Jun. 24, 1986, it is known ina continuous process to convey a superconductive bundle of thread,comprising an armoring thread, which is to be formed into cylindricalshape together with a metal band, which will be rolled around the bundleof thread in such a way that cavities are eliminated. Subsequent to seamwelding of the metal band for the purpose of forming a tube, the crosssection of the tube is reduced by at least 80% by swaging and finally itis rolled into square cross section. The superconductive threads aremanufactured by use of heat treatment of a suitable compound. The metalband is made of Cu or Al and the reinforcing threads are stainlesssteel, Mo or W.

The present invention also is directed to a method of manufacturing asuperconductive cable involving the steps of placing a continuous stringof superconductive ceramic material inside a tubular, closed metal cap.Thereafter, the closed metal cap is subjected to a forging process bymeans of swaging, hammering or an explosive process, all of which resultin the reduction of the cross-section of the cap to thereby tightlysurround and compress the enclosed materials. The superconductiveceramic material being of such a type that it achieves superconductiveproperties under specified heating conditions when it is surrounded by apowderous ceramic material the sintering temperature which is higherthan the reaction temperature of the composition of the core itselfwhich is comprised of superconductive ceramic material.

Due to the fact that the sintering temperature of the surroundingmaterial is higher than the reaction temperature of the solid substanceof the superconductive material it is ensured that around thesuperconductive core there will be a permeable layer, through which itis possible to introduce the maintaining atmosphere around the core.

Preferably, the composition forming the superconductive ceramic materialcore is introduced into the cap as a powder. During the forging process,the cross-section of the cap will be reduced and the powderouscomposition is sintered under pressure to form the superconductiveceramic material core.

Surprisingly it has proved that the necessary solid substance reaction,which normally takes a protracted heating under high pressure and with acontrolled composition of the surrounding atmosphere, may be attainedthrough a forging of the cap, when the forging takes place with such anintensity that the powder is compressed simultaneously with thereduction of the diameter of the tube. In practice the forging maypreferably be carried out through swaging, i.e. in a reduction machine,and it has proved that the desired reaction sets in quickly and that asintering of the superconductor takes place in such a way that thedesired characteristics are achieved even though several meters oftubular cap are advanced through the machine per minute of time.

According to the invention it is preferable that the tube is made from ahard and ductile metal, e.g. a corrosion resistant steel, which providesa relatively high resistance to the forging. It is particularlyimportant that the tube material does not show any tendency to absorboxygen, which may be liberated from the superconductive ceramicmaterial.

Thus a method is provided, in which the previously so problem-filledsintering process may be carried out in a fast and efficient way, thehitherto existing limitations on the length of the sintered stringsbeing eliminated.

In this connection it should be mentioned that from U.S. Pat. No.4,717,627 it is known to manufacture a fine-grained, superconductive ormagnetic material by means of the following procedure: On a first layerof solid material a second layer of powder and a third layer of solidmaterial are placed, and the assembly is placed in a sturdy container.Then a supersonic wave is transmitted through the first, the second, andthe third layer in the sequence mentioned, so that the second layer isheated to a temperature exceeding the melting point of the material bymeans of a shockwave pressure which is higher than 50 kBar. The melted,compressed layer will rapidly cool by heat transmission to the other twolayers.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in detail by means of an embodiment andwith reference to the drawing, which shows a section of asuperconductive cable according to the invention.

DETAILED DESCRIPTION

As shown in the drawing, the superconductive conductor or cableaccording to the invention comprises an outer metallic cap 1, in which acore 2 of a superconductive ceramic material is placed. In the spacebetween the core 2 and the cap 1, a diffusive ceramic material 3 isplaced, which has a higher sintering temperature than thesuperconductive core 2. The diffusive material makes it possible before,under and/or after the sintering of the superconductive material, tooptimize the surrounding atmosphere around the core, providing it withproperties which act to preserve or regenerate the core. In most casesthe problem is to control the contents of O₂, the superconductive corepreferably consisting of a ceramic material in Y-Ba-Cu-O category, butwhich comprises a great number of superconductive ceramic materials,which become superconductive at temperatures about and somewhat over theboiling point of liquid N. These materials may contain elements otherthan the above mentioned: Yttrium, barium, copper and oxygen.

The cable may be provided with means for maintaining a predeterminedatmosphere in the material 3, e.g. in the form of welded stubs. Thematerial 3 is a ceramic powder, which has a higher sintering temperaturethan the superconductive core 2. Preferably a temperature-resistantoxide of one of the ceramic oxides will be used, e.g. MgO, Al₂ O₃ orZrO₂.

In manufacturing the cable care must be taken that the cap closelysurrounds the core and the powder-like layer. This may be achievedfollowing the method according to the invention. In a special aspect ofthe method the necessary solid matter reaction, which transforms asuperconductive powder into a sintered superconductive ceramic string ofmaterial is achieved.

In addition, the core of the cable may comprise elements for mechanicalreinforcement of the core, for cooling thereof or for dividing it into anumber of parallel conductors.

A superconductive cable according to the invention and of a limitedlength can be manufactured in the following way: A tube of Inconel,which is a temperature-resistant and corrosion-resistant compound with ahigh content of Ni and corrosion-resistant compound with a high contentof Ni and having an outer diameter of 14 mm and an inner diameter of 11mm is filled with an intermediate layer of an isolating, ceramic powderAl₂ O₃ adjacent to the metal wall and a core of a powder whichconstitutes the superconductive core in the following way: Within theInconel tube a tube, made for example of glass is placed, said tubehaving an outer and an inner diameter of 8 mm and 6 mm respectively, andwithin this tube is placed a tube with an outer and an inner diameter of4 mm and 3 mm respectively. In the inner tube is placed a metal bar of athickness of 2 mm. By means of funnels two different kinds of powdersare dosed. By means of the first funnel Al₂ O₃ is added to theinterspace between the Inconel tube and the glass tube, and by help ofthe second the powder having the composition YBa₂ Cu₃ O_(x) and a powdergrain dimension of 0-60 is fed, said powder forming the superconductivestring, in the space between the inner tube and the metal bar. Firstly,approximately 5 cm Al₂ O₃ powder is filled into the outer opening whilethe outer glass tube is being used for stamping. Then the inner tube isdrawn up 1-2 cm, and superconductive powder is added which powder thenis stamped with the metal bar. Subsequently, the glass tube is againlifted and there is filled up and stamped until a height of 4-4.5 cm isachieved Again Al₂ O₃ powder is added, said powder being stamped etc.,until the tube is filled with powder. The tube is then sealed withon-welded terminals and subsequently forged to the specified diameter.In this case the tube was processed in a swaging machine therebyachieving a reduction in diameter from 14 mm to 9 mm. At the same time acorresponding pressing of the ceramic powder was achieved. The reductionof the diameter in the swaging machine took place with an advancingspeed of 1-2 m/minute.

The superconductive cable manufactured in this way was tested and thefollowing measurements carried out on the superconductive stringaccording to the formula YBa₂ Cu₃ O_(x) :

    ______________________________________                                        Dimension of crystallite                                                                            approx. 1300 Å                                      (measured by X-ray diffraction)                                               Dimension of crystallite                                                                            approx. 1300 Å                                      (measured by X-ray diffraction)                                               Porosity              approx. 1.5 Vol %                                       (measured by Hg-porosimetry)                                                  Skeleton density      approx. 5.55 g/ml                                       (measured by Hg-porosimetry)                                                  Positive indication of Meisner-effect at 77K.                                 ______________________________________                                    

After the heating treatment of the cable in 2 h at 930° C. thesuperconductive string has been investigated for shrinkage, and it wasfound that the string lies solidly embedded in the surroundingmaterials. Before the heating treatment it was found that a regularsintering of the Al₂ O₃ layer had not taken place, but it is found as acompact powder.

The powder, being used for making the superconductive string, had anormal composition for the forming of a superconductor of the typeY-Ba-Cu-0. However the same method will also be applicable in connectionwith other compositions of ceramic superconductors. Furthermore, it ispossible to support the sintering of the superconductor in the forgingprocess by means of heating to a temperature, which is below 900° C.said temperature being a common temperature for achieving sintering byheating alone. At such temperatures below 900° C. the surroundingceramic insulation will maintain its permeability, whereby it will bepossible also during the forging to regulate the atmosphere surroundingthe core.

In addition to swaging, which is used in the example, the forging maytake place by hammering or by an explosion-deformation. Furthermore, theforging may take place in such a way, that the tube achieves anotherform than the normally used circular form. Moreover, it is possible inthe core to embed reinforcing- or cooling elements and elements whichdivide the core into a number of parallel strings.

The manufacture of larger lengths of superconductive cables may beperformed by application of modifications of the technology which isknown from manufacturing of continuously filled, tubular packings. Whenthe various strings of powder have been placed in the tube forming thecap and stamped appropriately, the sintering of the core may be carriedout by swaging in particular, which allows treatment of even very largelengths of material.

We claim:
 1. Method of manufacturing an electric superconductive cablemade of a composition of one or more superconductive ceramic material,which under specific conditions of manufacture is made superconductiveand having a core located in a tubular, closed metal cap, comprising:introducing a powderous ceramic material between the core and the cap tosurround the core and subjecting the cap to a forging process by meansof swaging, hammering or explosion-deformation, to reduce the crosssection of the cap to tightly surround the materials in the cap, thesuperconductive ceramic material being of such a type that it mayachieve its superconductive properties in a solid material reactionunder heating, the sintering temperature of the powderous ceramicmaterial being higher than the sintering temperature of the compositionforming the core.
 2. Method according to claim 1, characterized in thatthe composition forming the core is introduced into the cap in the formof a powder, and forming the core by the forging process being carriedout with such an intensity that the cross section of the cap is reducedand the core is pressed and sintered during its formation.
 3. Methodaccording to claim 2, characterized in that in order to increase theeffect of the forging process, the cap is manufactured from a hard andductile metal.
 4. Method according to claim 3, characterized in thatbefore, during and/or after the forging process a protecting orregenerating atmosphere, is introduced into the powderous ceramicmaterial surrounding the core.
 5. A method according to claim 4,characterized in that the atmosphere is oxygen.
 6. Method according toclaim 2, wherein the cap is heated to a temperature below thetemperature at which sintering of the composition forming the core takesplace.
 7. Method according to claim 6, characterized in that before,during and/or after the forging process, a protecting or regeneratingatmosphere, is introduced into the powderous ceramic materialsurrounding the core.
 8. A method according to claim 7, characterized inthat the atmosphere is oxygen.
 9. Method according to claim 6characterized in that before, during and/or after the forging process aprotecting or regenerating atmosphere is introduced into the powderousceramic material surrounding the core.
 10. A method according to claim9, characterized in that the atmosphere is oxygen.
 11. A methodaccording to claim 2, characterized in that in order to increase theeffect of the forging process, the cap is of corrosion resistant steel.